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    SubjectRe: [PATCH v8 00/12] use nonblock mmc requests to minimize latency
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    On Tue, Jun 28, 2011 at 1:41 PM, Per Forlin <per.forlin@linaro.org> wrote:
    > How significant is the cache maintenance over head?
    > It depends, the eMMC are much faster now
    > compared to a few years ago and cache maintenance cost more due to
    > multiple cache levels and speculative cache pre-fetch. In relation the
    > cost for handling the caches have increased and is now a bottle neck
    > dealing with fast eMMC together with DMA.
    >
    > The intention for introducing non-blocking mmc requests is to minimize the
    > time between a mmc request ends and another mmc request starts. In the
    > current implementation the MMC controller is idle when dma_map_sg and
    > dma_unmap_sg is processing. Introducing non-blocking mmc request makes it
    > possible to prepare the caches for next job in parallel to an active
    > mmc request.
    >
    > This is done by making the issue_rw_rq() non-blocking.
    > The increase in throughput is proportional to the time it takes to
    > prepare (major part of preparations is dma_map_sg and dma_unmap_sg)
    > a request and how fast the memory is. The faster the MMC/SD is
    > the more significant the prepare request time becomes. Measurements on U5500
    > and Panda on eMMC and SD shows significant performance gain for large
    > reads when running DMA mode. In the PIO case the performance is unchanged.
    >
    > There are two optional hooks pre_req() and post_req() that the host driver
    > may implement in order to move work to before and after the actual mmc_request
    > function is called. In the DMA case pre_req() may do dma_map_sg() and prepare
    > the dma descriptor and post_req runs the dma_unmap_sg.
    >
    > Details on measurements from IOZone and mmc_test:
    > https://wiki.linaro.org/WorkingGroups/Kernel/Specs/StoragePerfMMC-async-req
    >
    > Changes since v7:
    >  * rebase on mmc-next, on top of Russell's updated error handling.
    >  * Clarify description of mmc_start_req()
    >  * Resolve compile without CONFIG_DMA_ENIGNE issue for mmci
    >  * Add mmc test to measure how performance is affected by sg length
    >  * Add missing wait_for_busy in mmc_test non-blocking test. This call got lost
    >   in v4 of this patchset when refactoring mmc_start_req.
    >  * Add sub-prefix (core block queue) to relevant patches.
    >
    > Per Forlin (12):
    >  mmc: core: add non-blocking mmc request function
    >  omap_hsmmc: add support for pre_req and post_req
    >  mmci: implement pre_req() and post_req()
    >  mmc: mmc_test: add debugfs file to list all tests
    >  mmc: mmc_test: add test for non-blocking transfers
    >  mmc: mmc_test: test to measure how sg_len affect performance
    >  mmc: block: add member in mmc queue struct to hold request data
    >  mmc: block: add a block request prepare function
    >  mmc: block: move error code in issue_rw_rq to a separate function.
    >  mmc: queue: add a second mmc queue request member
    >  mmc: core: add random fault injection
    >  mmc: block: add handling for two parallel block requests in
    >    issue_rw_rq
    >
    >  drivers/mmc/card/block.c      |  505 ++++++++++++++++++++++++-----------------
    >  drivers/mmc/card/mmc_test.c   |  491 ++++++++++++++++++++++++++++++++++++++--
    >  drivers/mmc/card/queue.c      |  184 ++++++++++------
    >  drivers/mmc/card/queue.h      |   33 ++-
    >  drivers/mmc/core/core.c       |  167 +++++++++++++-
    >  drivers/mmc/core/debugfs.c    |    5 +
    >  drivers/mmc/host/mmci.c       |  147 +++++++++++-
    >  drivers/mmc/host/mmci.h       |    8 +
    >  drivers/mmc/host/omap_hsmmc.c |   87 +++++++-
    >  include/linux/mmc/core.h      |    6 +-
    >  include/linux/mmc/host.h      |   24 ++
    >  lib/Kconfig.debug             |   11 +
    >  12 files changed, 1345 insertions(+), 323 deletions(-)



    Boot tested on Omap4430 Blaze board.

    Tested-by: Sourav Poddar<sourav.poddar@ti.com>
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